1. Field of the Invention
This invention relates to an apparatus for insertion of flow control devices into plastic pipe through which a fluid is flowing without removing the plastic pipe from service or otherwise interrupting flow through the plastic pipe.
2. Description of the Prior Art
Hot tapping of in-service pipelines is used to add valves, regulators, branch connections, and other devices to existing pipelines as well as to make in-service repairs on such pipelines. In hot tapping an in-service pipeline, it is necessary to cut an opening in the pipe for insertion of the valve or regulator, or for connection of a branch pipe. To prevent fluid within the pipe from escaping, a hot tap saddle is sealingly secured to the pipe around the portion of the pipe in which the hole is to be made. The saddle is provided with a gate valve or other means for providing access to the pipe without permitting fluid to escape from the pipe. A saddle suitable for such purpose is taught by U.S. Pat. No. 4,655,480.
Due to strength of material considerations, systems utilizing plastic pipe require considerations different from metal pipes when hot tapping. At least three techniques are currently known and used for hot tapping plastic pipes to insert a valve or branch connection, all of them having important limitations. The most frequently used technique involves squeezing the plastic pipe to shut down flow through the pipe after which the valve o branch connection can be installed. However, squeezing may affect the long term strength of the pipe. In addition, a potential safety hazard is created due to static electricity generation. And finally, flow through the pipe is interrupted, a less than desirable condition.
A second known technique is more direct and involves th installation of commercially available valves to be actuated through valve boxes when the need arises. However, the cost of such a valve installation is high. In addition, retrofitting of plastic pipe systems with known valves requires two squeeze off operations, thereby resulting in flow interruptions and safety hazards due to a generation of static electricity.
A third technique is a commercially available line stopper developed specifically for plastic pipe. However, such a line stopper is intended primarily for temporarily stopping flow in a plastic pipe and is not suitable for use as a sectionalizing valve. In addition, known devices of this type are also limited to pipe sizes of four inches and larger due to the complicated nature of their mechanisms. Thus, such stoppers are not suitable for plastic pipes having diameters less than four inches.
Hot tapping of a full bore valve in a plastic pipe having a fluid flowing therethrough is taught by U.S. Pat. No. 4,628,955, U.S. Pat. No. 3,687,166, U.S. Pat. No. 4,177,827, and U.S. Pat. No. 4,119,115. The '955 patent teaches a valve having a two part valve body which surrounds the pipe, the valve body parts being secured together by mechanical fasteners. The valve is equipped with a gate on a valve stem which can be reciprocated to open and close the valve. The gate has two gate sections, each having on its lower or leading edge a sharp knife edge which shears the pipe wall when the gate is initially advanced to the closed position by turning the valve stem and ultimately severs the pipe. As the gate approaches the fully closed position, the leading edges of the gate sections encounter a camming surface which acts to mechanically depress the gate sections against their seats to assure a tight closure of the valve. The '166 patent discloses a control valve having a split housing disposed around a portion of pipe and sealed against the pipe by O-rings, solder, sealing compound, plastic film, or other suitable sealing material. Coupled to the lower end of the valve stem is a body, the lower end of which is provided with a pair of cutters such that when the valve stem is turned, the body is lowered toward the pipe and the cutting edge cuts through the pipe wall to completely sever the pipe. The remainder of the body is a solid wedge gate valve which can be used to control the flow of fluid through the pipe. The '827 patent teaches a flow control apparatus having a fluid tight housing mounted around a pipe, a shear, an actuator means for cutting transversely through the pipe wall and a second independent actuator means for moving a surface associated with the shear into engagement with the sealing surface around the pipe. Finally, the '115 patent teaches an apparatus for cutting into and stopping the flow of fluid in a plastic pipe having a cutting blade that is forced downwardly into a pipe by hydraulic force, the cutting blade being slightly larger than the internal diameter of the pipe such that when the blade is fully lowered into the pipe, the flow of fluid into the pipe is stopped.
U.S. Pat. No. 4,552,170 teaches a valve insertable into an in-service pipe having two halves clamped around the pipe and a cutter connected to a valve handle which is rotated to drive the cutter through the pipe wall and position an expandable seal across the pipe bore, thereby terminating flow through the pipe Retraction of the seal and cutter opens the valve to reinitiate fluid flow and the valve can thereafter be used as a conventional shut off valve.
U.S. Pat. No. 3,948,282 teaches a method for attaching a water shut off control valve to an in service pipe by attaching a T pipe around the water pipe, mounting a valve assembly on the T pipe assembly, attaching a hole-boring assembly to the valve assembly and boring a hole into the water pipe, closing the valve and subsequently removing the hole-boring assembly, thereby maintaining the water within the pipe. A second valve assembly is then mounted to the free end of the previously mounted valve assembly and the valve seat thereof inserted through the opening into the water pipe to shut off the flow of water.
U.S. Pat. No. 3,749,108 teaches a valve stopper for insertion into an in-service pipe having a rubber patch clamp used in combination with a slide gate valve assembly. See also U.S. Pat. No. 4,655,480 and U.S. Pat. No. 4,431,017 which teach methods and apparatuses for connecting a branch line to an in-service pipe line.
One of the problems associated with installation of a full bore valve into an in-service plastic pipe is the requirement that the gate valve provide an effective seal when the gate is in the lowered position. Known apparatuses which feature a combination pipe cutter and gate valve as taught by the '955 patent, the '166 patent, and the '827 patent require a secondary motion once the gate has been inserted into the pipe to seal the gate against the open pipe ends. In the '955 patent, the two halves of the gate are forced against the seats machined into the upper and lower housing sections of the valve body by camming surfaces. In the '827 patent, the secondary motion to achieve a seal is effected by the tightening of thrust screws against the gate which push the gate against seats machined into the upper and lower housings of the valve body. The '166 patent teaches two cutters which are initially spread apart to cut away a segment of the top of a pipe which cutters are forced together by sealing surfaces as the cutters pass through the pipe. In addition, each of the devices taught by the '955 patent, the '166 patent, and the '827 patent require the removal of a "ring" of the pipe section due to the secondary movement requirements of the gate. This requirement doubles the force necessary for cutting through the pipe over a cutter which employs a single shear having one cutting edge. The secondary movement of the gate in these devices may require substantial forces. The practical implementation of these devices may make the use of plastic for the housing material unacceptable.
Finally, the device taught by the '115 patent utilizes a blade with a width which is slightly larger than the internal diameter of the pipe such that when the blade is lowered into the pipe, a slot is generated in the inner wall of the pipe into which the blade fits to provide a seal. However, such an approach provides a substantial amount of stress concentration due to alteration of the pipe geometry. Such stress concentration does not occur with a completely severed pipe which results in essentially two free ends of pipe. In addition, the effectiveness of the seal between the blade and the groove is questionable because the groove in the pipe tends to open as the blade moves through the pipe, an effect of the displacement of material at the cutting edge. In addition, even the smallest amount of material which lodges in the groove will prevent sealing of the blade within the groove.